Polyaxial screw implants for assisting in fusion surgeries have been in use for many years. Typically, polyaxial screw motion implants allow motion of a body portion around a spherical screw head to allow the assembly to pivot. When individual screw assemblies are connected by a rod, it is common for the rod to be out of alignment with all of the screw assemblies. This problem increases in severity with an increasing number of screw assemblies that the surgeon is trying to connect to the same rod. Pivoting allows the assembly to compensate for this problem.
The main problems with polyaxial screws are package size and implant locking. Most implants are fairly large in size in order to allow enough force to be exerted against the spherical screw head and rod without the implant flexing under the load. For all of these implants, and a few that are small in size, the locking component is complex and often involves the use of threads. Set screws and threaded nuts are commonly used. Finding a thread inside the implantation site is difficult and creates issues such as cross-threading. Other concepts have been invented, but as a combination of effective locking and small package size, room for improvement exists.
In the majority of the prior art screw assemblies, the screw head portion of the screw is spherical in form, such that it can rotate in all directions. There are a few exceptions to the rule, but the result is essentially the same. For example, U.S. Pat. No. 7,186,255 to Baynham et al. shows a screw portion having a spherical head separated from the screw shank. However, once assembled, the screw is no different than the other prior art. U.S. Pat. No. 7,163,539 to Abdelgany et al. shows a screw with a partial spherical head that has been hollowed. However, the concept is still providing a spherical head. This approach also has limitations in the amount of allowable rotation in all directions, as the edges of the head contact the rod when the head is rotated. U.S. Pat. No. 5,891,145 to Morrison et al. utilizes a polyaxial screw head with a disk-like projection resting in a taper. This complex design requires compression of the disk-like projection in a taper. The difficulties in this are high because when the screw head rotates, only a small portion of the disk-like projection is in contact with the taper, thereby making it almost impossible to lock the angulation of the screw in position.
U.S. Pat. No. 5,584,834 to Errico et al. discloses that a taper is required on the exterior surface of an external ring to engage and create sufficient pressure to lock a screw head. In addition, this design is restricted to a curvate screw head. The external locking collar is also tapered on its inside aspect and it is a requirement of the design. U.S. Pat. No. 5,578,033 to Errico et al. discloses a spherical head of a hook that is used in place of a spherical head of a screw. The implant loads the rod component from the side, with a locking collar sliding over a tapered lower section. Both of these prior art Errico patents require the use of a locking nut to lock the assembly in its final position. U.S. Pat. No. 5,586,984 to Errico et al. discloses an external taper on a body portion of a locking ring to clamp the tapered section against a round screw head. This is also done by advancing a threaded nut.
United States Publication No. 2007/0173819 to Sandlin et al. shows a locking cap design to engage a tapered upper portion of a body. A tapered portion can be used to cause contouring around a rod, which has been shown in other prior art patents. The cap also engages in such a way as to be fully locked or not locked at all. There are no provisions for partial locking.